Erik:
You may need to provide a bit more information, but if USB is the environment then I would say that you want to make your device look like an ATA device and respond to the commands that such device will receive when it is connected to a host platform. My guess is that the easiest way to achieve this would be to organize the device at the interface side to look like the block device it is and map the 512 byte blocks into a RAM device. On the device core side take the memory blocks and make them hold the core data. In order for the blocks to make ultimate sense to the PC side it will want to be mapping the data to your device as if it is a file. As such your core side is going to have to deal with this and be aware of this file system overhead.

The job to be done has many attributes in common with the process of trying to embed a compact flash card into your hardware and read/write data to it in a manner that such card, when removed and plugged into a PC, that the host OS can understand the resulting block structure as valid file data.

Another thing is that this does share some similarity to some of the USB MP3 player projects that one sees on the WEB wherein the developer may have provided the ability to "download" MP3 files from a PC over the USB.

The work size is probably going to be rather substantial. I know that in the looking and searching I have done on this subject that some developers have chosen shortcuts in how they implemented the pseudo ATA functionality to make the taget development easier. What I mean is that they implement a subset of ATA in a manner to such as only respond to some of the host commands. Maybe they make their device look just like a RAW device without a file system and then write a raw level access program at the PC side. Or maybe they make the device look like a single file that fills up the whole "disk" and can only be read and written in place as blocks. Etc etc

Michael Karas

PS...personally I would think that making your device look like a stream device is probably easier.
MJK